Optimize MethodData layout

pahole reports MethodData can be compacted a little:

```
class MethodData : public Metadata {
public:
        /* class Metadata <ancestor>; */ /* 0 0 */

        /* XXX 8 bytes hole, try to pack */

        class Method * _method; /* 8 8 */
        int _size; /* 16 4 */
        int _hint_di; /* 20 4 */
        class Mutex _extra_data_lock; /* 24 112 */
        /* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
        class CompilerCounters _compiler_counters; /* 136 80 */
        /* --- cacheline 3 boundary (192 bytes) was 24 bytes ago --- */
        intx _eflags; /* 216 8 */
        intx _arg_local; /* 224 8 */
        intx _arg_stack; /* 232 8 */
        intx _arg_returned; /* 240 8 */
        int _creation_mileage; /* 248 4 */
        class InvocationCounter _invocation_counter; /* 252 4 */
        /* --- cacheline 4 boundary (256 bytes) --- */
        class InvocationCounter _backedge_counter; /* 256 4 */
        int _invocation_counter_start; /* 260 4 */
        int _backedge_counter_start; /* 264 4 */
        uint _tenure_traps; /* 268 4 */
        int _invoke_mask; /* 272 4 */
        int _backedge_mask; /* 276 4 */
        short int _num_loops; /* 280 2 */
        short int _num_blocks; /* 282 2 */
        enum WouldProfile _would_profile; /* 284 4 */
        int _jvmci_ir_size; /* 288 4 */

        /* XXX 4 bytes hole, try to pack */

        class FailedSpeculation * _failed_speculations; /* 296 8 */
        int _data_size; /* 304 4 */
        int _parameters_type_data_di; /* 308 4 */
        int _exception_handler_data_di; /* 312 4 */

        /* XXX 4 bytes hole, try to pack */

        intptr_t _data[1]; /* 320 8 */

        /* size: 328, cachelines: 6, members: 27 */
        /* sum members: 312, holes: 3, sum holes: 16 */
        /* last cacheline: 8 bytes */
};

```